1. Field of the Invention
The present invention relates to a generation method for an output clock with a duty ratio, and to a clock generation circuit corresponding to it.
2. Description of the Related Art
Crystal oscillators are used for generation of the output clock, especially at relatively high frequencies. They normally have a duty ratio, a time ratio between the high level and the low level (see page 4, lines 18 and 19 of the originally submitted description) of approximately 1:1.
According to the reference document “Specialist expressions for word processing and data processing—dictionary and glossary”, issued by IBM, the expression “duty ratio” means the “ratio of the pulse duration to the period duration of a pulse”.
Widely differing signal forms are known from the relevant specialist literature. For example, a so-called “square-wave pulse sequence” is known from “Kenneth W. Cattermole: Signale und Wellen—Mathematische Grundlagen und Anwendungen” which may be translated as “Signals and waves—Mathematical principles and applications”, VCH Verlagsgesellschaft, 1998, (page 54; and elsewhere), and a so-called “period square wave” (page 180; and elsewhere) as well as a so-called “symmetrical period square wave” (page 432; and elsewhere) are known from “Oppenheim, Willsky: Signale und Systeme—Lehrbuch” which may be translated as “Signals and systems—Textbook”, VCH Verlagsgesellschaft, 1999. The minimum amplitude of such a symmetrical period square wave need not necessarily be less than zero, as described in the textbook; the minimum amplitude—the low level—is frequently equal to, or approximately equal to, zero. However, this corresponds to a simple shift of the otherwise unchanged signal form along the ordinate.
In the terminology of the present application, the expression “duty ratio” means the ratio between the duration of the high level and the duration of the low level of an at least essentially symmetrical periodic square wave. In the terminology of this application, exactly symmetrical period square waves have a duty ratio of 1:1 (see above) since, owing to the symmetry of the square wave, the duration of the high level in fact corresponds exactly to the duration of the low level in one period of such a symmetrical periodic square wave.
Such symmetrical periodic square waves are frequently the output signal from a clock generation circuit, so that the symmetrical periodic square wave represents the clock signal for a circuit or an appliance.
In the prior art, considerable problems frequently occur with regard to inadvertent radio-frequency radiated emissions. Various measures are necessary in order to comply with the legal regulations and/or to avoid disturbing interference.
For example, in the case of personal computers, it is known for the clock generation to be designed such that the frequency is continuously varied by a small amount. If, for example, a crystal oscillator has a nominal frequency of 50 MHz, then, by way of example, its actual frequency is varied in the range between 49.9 MHz and 50.1 MHz. The frequency of the interference spectrum, with the harmonics contained in it, is thus continuously shifted. This results in the interference spectrum being distributed over a wider range, hence resulting in the mean interference power being reduced—in comparison to a fixed frequency. However, the radiated interference power that is emitted is actually constant in a relatively narrow frequency band about one harmonic.
The procedure in the prior art itself leads to a considerably reduction in the mean interference power per frequency. However, this requires the clock frequency to be varied.
In some applications, data transmission methods at a fixed frequency and synchronization methods may be mentioned by way of example, it is not, however, permissible to vary the frequency in such a way. In situations such as these, the method for the prior art cannot be used, by virtue of the system.